A. Field of the Invention
This invention relates to a device to transmit motion from one rotating shaft to another, and more particularly to such a device in which there is a constantly changing ratio of angular velocities between the shafts.
B. Prior Art
Various types of linkages exist for transferring rotational motion from one shaft to another, including gears, belts and pulleys, rollers and the like. In general, these provide a fixed transmission ratio between the two shafts; that is, the angular velocity of the second shaft divided by that of the first is constant. Some linkage devices, such as cams and followers, provide variable transmission ratios, but they generally work only through a one-revolution cycle, or through several revolutions with relatively complex design.
When a wound flat web, such as magnetic tape or projection film, is moved from one reel to another, the ratio of the rotational speeds of the two shafts upon which the reels are mounted constantly changes because of the constantly changing diameters of the web on the two reels. On the other hand, it is often desired to move the web at a constant linear speed past some point, such as a record/playback head on a tape recorder or the object lens on a movie projector. For many purposes, it is also important to maintain a constant tension on the web as it moves, and while it is at rest. Constant tension is particularly important when moving a high-density digital magnetic tape over a read/write head, since tension variations can cause undue speed fluctuations at the head.
In some types of magnetic tape drives, the tape is driven at constant speed by the use of a capstan device directly driving the tape. However, this arrangement is unsuitable for high-speed, high-density digital computer tape drives. Therefore, such tapes are generally driven by individual reel motors with control circuits that operate to maintain constant linear speed. In such devices, opposing torques must be applied to the reel shafts to produce the desired web tension. However, this adds to the cost and complexity of the equipment, and requires an expenditure of energy to maintain tension even when the tape is at rest.
A tape cassette has been used which includes an internal means of providing tape tension. This is accomplished by providing an elastic band in the cassette, running in contact with the outside of the tape on the two reels, which acts as a driving and tensioning means. As the band is driven by an external motor, the band both drives the tape and keeps it tight. However, this requires the use of special cassettes, each one of which requires the elastic band device. Furthermore there have been problems maintaining constant tension when the tape is frequently stopped and started or reversed.